(a) Field of the Invention
The present invention relates to a receiving method and a receiving apparatus of a communication system having multiple antennas. More particularly, the present invention relates to a receiving method and a receiving apparatus of an orthogonal frequency division multiplexing access (OFDMA) system.
(b) Description of the Related Art
A basic principle of an orthogonal frequency division multiplexing (OFDM) system is that input data of a high data rate are arranged in parallel by the number of carriers as data of a low data rate and are transmitted on each carrier. At this time, since symbol blocks of subcarriers of a low data rate are increased, a relative distortion on a time phase is decreased due to a multipath delay spread, and longer guard intervals than a channel delay speed are inserted between all the OFDM symbols so that inter-symbol interference may be removed.
Since such an OFDM modulation/demodulation uses a plurality of subcarriers, it is difficult to design hardware as the number of subcarriers has been increased. In addition, it is difficult to maintain orthogonality between subcarriers and to realize it in an actual system. Discrete Fourier transform (DFT) may solve such problems, and many DFT operations may be decreased by using a fast Fourier transform (FFT).
Such an FFT is performed by continuously disintegrating DFT of a length N by DFT of a smaller length in time decimation or frequency decimation. In this manner, the number of multiplications among the operations of DFT is decreased from N2 times to NlogN times. For efficient operation of such an FFT, Radix-2 DIT (decimation in time) FFT and DIF (decimation in frequency) FFT, Radix-4 DIT FFT and DIF FFT, and Radix-22 DIT FFT and DIF FFT are used.
Meanwhile, an OFDM transmission method of a multiple antenna system may include a MIMO (multiple-input multiple-output) method such as using a smart antenna for performing beam forming according to a structure of the multiple antennas. Various analyses have been performed, for example a post-FFT in which FFT is performed for the respective antennas and then a weight value (weight factor) is multiplied, and a pre-FFT in which the weight values are multiplied and combined in a time domain and then one FFT is performed.
However, since a multiple access scheme using the OFDMA is structured such that an entirely-used subcarrier is allocated as each subscriber unit and transmitted as each subscriber unit, it may be not used for the pre-FFT in which the weight values are multiplied in a time domain.
Meanwhile, a multiple antenna receiver for the conventional wideband wireless communication performs FFT as many times as the number of antennas on the OFDM transmission using the multiple antennas. At this time, the FFT is performed as many times as the number of antennas after discovering a combined time synchronization and frequency synchronization using each received signal. In addition, after the FFT is performed, a channel is estimated and a weight value is calculated and combined using pilots of each received signal path and band.
That is, when performing an FFT for the respective antennas, the time synchronization and frequency synchronization as important factors of the performing of FFT are not obtained for the respective antenna receiving paths, but the FFT is performed for the respective antenna receiving paths after obtaining the time synchronization and frequency synchronization by combining the same.
In other words, the OFDMA demodulation may be realized by FFT, and the OFDMA performs FFT for each antenna in the multiple antenna system so that a signal of a subscriber station of an uplink may be recovered. However, since FFT is performed as many times as the number of antennas when the number of antennas is increased, an FFT scheme for performing FFT for the respective antennas has a complexity-increase problem.